1. Field of the Invention
This invention relates generally to spectral hole burning data storage systems and more specifically to such a system which uses phase modulation.
2. Description of the Prior Art
There are inhomogeneously broadened materials which have broad optical absorption spectrums. At low temperatures, these materials exhibit spectral hole burning phenomenon. This phenomenon involves the use of a laser to photochemically modify a material's optical absorption spectrum at selected frequencies. The atoms at these frequencies have been altered so that they no longer absorb light at these frequencies. The resulting optical absorption spectrum of the material appears to have frequency dependent holes in it, hence the name of spectral hole burning.
Various schemes have been proposed to use spectral hole burning to store data. The simplest scheme is frequency domain optical recording and involves using discrete frequency bands as digital data storage positions. The material's optical frequency spectrum is divided into a large number of small data frequency bands. Each frequency band corresponds to a bit position. A frequency variable laser is used to selectively burn holes into the different data frequency bands. If a band has a spectral hole, it represents a digital "zero," and if not, it represents a digital "one." Alternatively, the representations may be reversed. To read the information, a laser selectively interrogates each data band and if a spectral hole is detected by the optical detector it represents a "zero." If a spectral hole is not detected, it represents a "one." The spectral hole is detected by measuring the amount of light which passes through the material at the various data bands. The amount of data stored in any particular spot in the material depends on the ratio of the width of the material's absorption band (inhomogeneous linewidth) to the bandwidth of the individual holes (homogeneous linewidth). This varies depending upon the material, but there have been reports of storing up to approximately 2000 bits of information in a single optical spectrum. In other words, a single spot on the material can hold up to 2000 data bits, each bit recorded at a different frequency.
Examples of frequency domain spectral hole burning include U.S. Pat. No. 4,101,976 issued Jul. 18, 1978 by Castro, et al.; U.S. Pat. No. 3,896,420 issued Jul. 22, 1975 by Szabo; U.S. Pat. No. 4,297,035 issued Oct. 27, 1981 by Bjorklund; U.S. Pat. No. 4,306,771 issued Dec. 22, 1981 by Bjorklund; U.S. Pat. No. 4,533,211 issued Aug. 6, 1985 by Bjorklund, et al.; U.S. Pat. No. 4,158,890 issued Jun. 19, 1979 by Burland; W. Lenth and W. E. Moerner, "Grated Spectral Hole Burning Frequency Domain Optical Recording," Optics Communications, Vol. 58, No. 4, Jun. 15, 1986; and M. Yoshimura, et al., "Photochemical Hole Burning of Anthraquinone Derivatives in Acrylic Polymers," Chemical Physics Letters, Vol. 143, No. 4, Jan. 22, 1988. One problem with these systems is that it is difficult to reliably detect the small changes in light intensity as the laser sweeps the frequencies at higher speeds.
Time domain spectral hole burning is another type of data storage method. This technique uses a selectively pulsed laser to record data. The molecules of the inhomogeneously broadened material are first prepared for writing by the application of a powerful reference pulse which has a sufficiently broad frequency spectrum as to excite substantially all of the atoms in the material to an excited energy level. These reference pulses may be generated in three different ways: as a short pulse, (see W. Babbitt and T. Mossberg, "Time Domain Frequency Selective Optical Data Storage in a Solid State Material," Optics Communications, Vol. 65, No. 3, Feb. 1, 1988); or as a frequency swept or chirped pulse, (see Y. Bai, et al., "Coherent Transient Optical Pulse Shape Storage/Recall Using Frequency-Swept Excitation Pulses," Optics Letters, Vol. 11, p. 724, November 1986); or as a pseudo-random phase modulated pulse, (see J. Zhang, et al., "Use of Phase Noisy Laser Fields in the Storage of Optical Pulse Shapes in Inhomogeneously Broadened Absorbers," Optics Letters, Vol. 16, No. 2, Jan. 15, 1991).
The reference pulse is followed by a train of optical data pulses which are of lower intensity and centered at the same wavelength as the reference pulse. These data pulses represent the digital data delivered in a serial manner. This data sequence must be transmitted a short time after the reference pulse. The spectral components of the data sequence interact with the previously excited atoms and create a population grating which is stored in the material as spectral holes.
The information is read by applying a single reference pulse to the material. This reference pulse is similar to the first reference pulse which was used to record the information and excites the atoms in the material. However, there is now a population grating in the material which results in an excess of absorbing atoms at certain frequencies which are resonant with the spectral components of the data sequence. These excess absorbing atoms interact coherently to generate a photon echo. The photon echo is comprised of a series of time separated pulses which match that of the original data sequence. This photon echo is detected and reconverted into the original electrical data signal. However, the photon echo is exponentially decreasing in intensity and the signal waveforms are irregular, which make their detection difficult and expensive.
Examples of time domain systems are shown in T. Mossberg, "Time Domain Frequency Selective Optical Data Storage," Optics Letters, Vol. 7, No. 2, February 1982; M. Mitsunaga, et al., "248 Bit Optical Data Storage in Eu3+:YAlO.sub.3 By Accumulated Photon Echo," Optics Letters, Vol. 15, No. 3, Feb. 1, 1990; M. Mitsunaga, et al., "Effects of Hyperfine Structures on an Optical Stimulated Echo Memory Device," Optics Letters, Vol. 11, No. 5, May 1986; M. Mitsunaga, "CW Photon Echo: Theory and Observations," Physical Review, Vol. 42, No. 3, Aug. 1, 1990; R. M. McFarlane, et al., "Sub-KiloHertz Optical Linewidths of the 7Fo.fwdarw.5Do Transition in Y.sub.2 O.sub.3 :Eu3+," Optics Communications, Vol. 39, No. 3, Oct. 1, 1981; and N. N. Akhmedieve, "Information Erasing in the Phenomenon of Long Lived Photon Echo," Optics Letters, Vol. 15, No. 18, Sep. 15, 1990.
What is needed is a spectral hole burning data storage system which has a fast access time, high data density, and ease of data detection.